In recent years, there has been developed a so-called “active stabilizer system”, in detail, a stabilizer system in which stiffness or rigidity of a stabilizer bar is changed for thereby effectively controlling or restraining roll of a vehicle body. For instance, JP-A-2002-518245 and JP-A-2000-71738 disclose such a stabilizer system. Each of the disclosed stabilizer systems is equipped with an actuator which has an electric motor and which changes the stiffness of the stabilizer bar. By controlling the operation of the actuator, the roll of the vehicle body is effectively restrained or suppressed.